Soil nitrogen emissions in a changing climate: Lessons from the Colorado Plateau

Background/Question/Methods

In arid ecosystems, gaseous N fluxes can be a substantial proportion of annual N loss, directly affecting its’ long-term bioavailability.  Therefore, an essential part of understanding the impact of global climate change on arid regions is evaluating how the magnitude and composition of reactive N loss will respond to future environmental conditions. In addition to rising temperatures, global models predict alterations in the amount and timing of rainfall, with one prediction for the Colorado Plateau being an increase in the frequency of small summer precipitation events. In this study, we explored the effects of both increases in temperature and changes in precipitation patterns on soil N emissions from the Colorado Plateau. We measured fluxes of reactive N gases (NO, NO_y and NH₃) at a long-term manipulation experiment that included a warming treatment that raised soil temperature by 2 °C and an irrigation treatment that increased the frequency of small (< 5 mm) summer rain events. To assess the spatial and temporal variability in ecosystem responses to these climate change factors, we made measurements across 3 seasons (winter, spring and summer) and across the dominate surface cover types including in plant interspaces and beneath the shrub Atriplex sp.

Results/Conclusions

Increases in soil temperature and changes summer precipitation patterns altered N loss dynamics from the Colorado Plateau. During the winter and spring, a 2 °C increase in temperature nearly doubled total reactive N efflux from soils associated with Atriplex. In the summer, extremely high natural soil temperatures (40-80 °C) meant that the heat lamps were unable to raise soil temperatures, however natural variation in soil temperature did significantly influences rates of reactive N efflux. There was a significant positive relationship between soil temperature and N emissions, with variation in soil temperature explaining 40% of the variation in total reactive N fluxes. Changes in summer precipitation patterns had both an immediate and a lasting impact on soil N fluxes. Summer rainfall yielded large pulses of reactive N efflux, with fluxes averaging 29.5±10.3 ng N m^-2s^-1. Additionally, soils that received increased frequency of small (< 5mm) summer rain events, exhibited larger post-rain pulses of reactive N gases compared to control soils throughout the year. Taken together, these results suggest that changing precipitation patterns coupled with rising temperature has the potential to alter the N balance of arid ecosystems by increasing reactive N gas efflux.